The present invention relates to sulfur dioxide analyzers and, more particularly, to an improved sulfur dioxide analyzer having a generator for generating a sulfur dioxide containing standard gas from elemental sulfur.
In measuring the sulfur dioxide content of a sample gas of unknown composition, one widely used technique involves measuring the quantity of fluorescent radiation emitted by the same gas at wavelengths that are known to be characteristic of sulfur dioxide. This fluorescence is ordinarily stimulated by illuminating the sample gas with ultraviolet radiation through a first band pass filter having a pass band calculated to selectively excite sulfur dioxide molecules. The fluorescent radiation emitted by these excited sulfur dioxide molecules is then ordinarily measured with a photomultiplier tube that is exposed to the sample gas through a second band pass filter having a pass band corresponding to the frequency of sulfur dioxide fluorescence.
In calibrating sulfur dioxide analyzers of the above-described type, it has been the practice to supply the analyzer with a standard or span gas of known sulfur dioxide content from sources of one of two types. One type of source includes external storage tank of a compressed sulfur dioxide containing gas of known composition. While this arrangement operated satisfactorily, it has associated with it the expense and inconvenience of obtaining, installing, and maintaining a storage tank for compressed gas. As is well known, such tanks are bulky, heavy and subject to leakage.
Another type of source for the sulfur dioxide containing standard gas includes permeation tubes. The latter tubes typically comprise a body of porous material which has been impregnated with sulfur dioxide, and which is designed to gradually release the sulfur dioxide into a gas stream that is caused to flow thereover. While permeation tubes are convenient and inexpensive, they are also subject to problems that render their use unattractive. A major one of these is the relatively short time within which the permeation tube becomes functionally exhausted, i.e., provides a gas stream with less than the required quantity of sulfur dioxide. As a result, the use of permeation tubes under any but the most closely monitored conditions can lead to a progressive loss in the accuracy of the calibration upon which all operational measurements are based.
Another problem with permeation tubes is the instability of their sulfur dioxide output. As is well known, permeation tubes are sensitive to changes in temperature, humidity and gas flow rate. Since a given set of these conditions may not be accurately reproducible during successive calibrations, the data obtained after such calibrations may include differences not related to differences in the sulfur dioxide content of the sample gas. Thus, the use of permeation tubes can result in measurements containing significant errors.